1. Field of the Invention
This invention relates to a plasticizing apparatus for use in an injection molding machine, which apparatus comprises a plasticizing cylinder having an inside peripheral surface, which defines a cylindrical plasticizing chamber, which has a conical portion leading into a centrally disposed discharge passage, a rotatable feed screw, which is disposed in the plasticizing chamber and feeds the plasticized plastic material into a portion of the plasticizing chamber in front of the feed screw as the latter yields rearwardly, and is operable to perform a discharge stroke, by which the plastic material is injected into the injection mold of the injection molding machine.
In connection with such a plasticizing apparatus the term "high-pressure space" is used herein to describe the space which during the injection of the plastic material into the injection mold is disposed in front of the backflow-preventing valve and which is hermetically sealed from the "low-pressure space". The maximum volume of that high-pressure space is determined by the rearmost initial position of the feed screw. A distinction must be made between the rearmost initial position and the instantaneous rear initial position of the feed screw; the latter position will be determined by the quantity of plastic material which is to be discharged when a given molding is to be made and varies in dependence on the size of the moldings because only the quantity of plastic material which is required to make a given molding will be accumulated in each injection molding cycle in front of the feed screw.
2. Description of the Prior Art
The previously employed plasticizing cylinders are made as one-piece hollow cylinders (see, e.g., U.S. Pat. No. 4,886,439) and are closed at their forward end by a nozzle body, which has radial and axial sealing surfaces in contact with mating sealing surfaces of the plasticizing cylinder. The joint between the nozzle body and the plasticizing cylinder adjoins the high-pressure space at the inside peripheral surface of the plasticizing cylinder. The length of the circular line on which said joint is subjected to the pressure in the high-pressure space during the discharge of the plastic material is relatively large because it is determined by the inside diameter of the high-pressure space. The pressure built up in the high-pressure space during the discharge stroke may be as high as 2,500 bars or more and is applied to the high-pressure side of said joint throughout its length. In order to ensure an adequate seal between the nozzle body and the cylinder, the nozzle body must axially be forced against the sealing surfaces of the cylinder under an extremely high contact pressure, which is usually applied by an axial movement of a screw-threaded element. To build up said contact pressure, a very high torque must be exerted on the screw-threaded element and such torques can be exerted only by considerable manual forces and/or by means of relatively large tools. It must be borne in mind that an exchange of nozzles will have to be performed in the premises of the user of the injection molding machine either by an employee of said user or by a service technician of the manufacturer of the machine. An exchange of the nozzle will give rise to problems if highly skilled labor and suitable tools are not available, as may particularly occur in developing countries. Injection moldings are increasingly required to meet certain specifications but this requires the use of plastic materials or mixed plastic materials which can be injection-molded only with special injection molding technologies. Such plastic materials or mixed plastic materials which are required may release gases under the extreme pressure built up in the high-pressure space if said gases can escape through the joint between the cylinder and nozzle body.
In general, the nozzle body must be relatively long, as is also the case in plasticizing units as disclosed in U.S. Pat. No. 4,886,439, so that the discharge passage is long too because a considerable part of the axial length of the nozzle body is required to interlock the nozzle body with the cylinder and another axial length portion of the nozzle body is required for the attaching of retaining means for a thermocouple and/or for a valve needle. A relatively long discharge passage will result in a correspondingly high pressure drop in the plastic material as it is discharged.
Another plasticizing apparatus known from U.S. Pat. No. 4,842,507 comprises a bipartite plasticizing cylinder, in which a rear cylinder section, which is relatively small in outside and inside diameter, extends into a forward cylinder section, which is relatively large in outside and inside diameter, and the joint between the two cylinder sections adjoins the high-pressure space throughout the injecting operation.